Endless track-driven vehicles are commonly used off-road in difficult terrain conditions such as in mud, snow, sand and tundra. For example, tracked vehicles are used in snow for grooming ski slopes and snow mobile trails, for transporting skiers to back-country slopes, for ski resort maintenance work, and for utility company maintenance work.
Tracked vehicles generally are of two types. Many are two-tracked, in which a pair of endless track units, one on each of the opposite sides of the vehicle, support and drive the vehicle. The other type is four-tracked, in which four separately driven and independently suspended track units, two in front and two in the rear, support and drive the vehicle.
Four-tracked vehicles have certain advantages over two-tracked vehicles under extreme conditions such as on steep slopes and in very rough terrain because of the flexible independent suspension of the four-tracked units and the constant power available to all four-tracked units, even while turning. Unlike a two-tracked vehicle, which relies on the differential speed of the two tracks for turning, the four-tracked vehicle steers much like a wheeled vehicle. Its endless track units can be physically turned for steering.
Despite the advantages of four-tracked vehicles over two-tracked vehicles under extreme terrain conditions, the nature of four-tracked vehicles is such that there are several inherent problems with prior designs.
In many tracked vehicles the lower, ground engaging, run of the track has been supported by a plurality of single element guide wheels disposed substantially in line longitudinally of the track and generally engaging only a centralized region of the track. Although this system, with a single line of guide or support wheels, functioned adequately, it was found that substantial deflection of the track on opposite sides of the guide wheels was occurring. This deflection was caused generally by the high point load at the center of the lateral dimension of the track which could lead to premature failure of elements in the track due to high cyclical stresses.
Further, although inwardly directed lugs may have been provided on the track to engage opposite sides of the single guide wheels, it has been found that periodically during operation and turning the guide wheels may move out of the proper track between the lugs and misalignment of the track occurs.
Further, in prior devices with a single centrally located guide wheel the upward deflection which occurred in the track toward opposite sides of the wheel could allow the track, when moving along a side hill, to slide somewhat sideways down a hill, rather than maintaining a stable line of travel as desired.